The present invention, relates to the field of lithium-ion battery technology, in particular anode materials suitable in this regard and their production.
The present invention relates in particular to a method for manufacturing an electrically conductive, porous, silicon- and/or tin-containing carbon material as well as the carbon material itself produced in this way and its use in particular for production of anode materials, preferably for lithium-ion batteries.
In addition, the present invention relates to anode materials containing the carbon material according to the invention and/or using the carbon material according to the invention as well as batteries, in particular lithium-ion batteries comprising these anode materials.
Lithium-ion batteries are characterized by very high energy densities. They are thermally stable, supply a constant voltage over the duration of the discharge time and do not have a so-called memory effect. Such batteries are known in the prior art in the form of traditional batteries for a single use as well as in the form of rechargeable batteries.
Lithium-ion batteries generate the electromotive force by displacement of lithium ions. In the charging process, positively charged lithium ions migrate through the electrolyte from a cathode between the graphite planes of an anode, while the charging current supplies the electrons over the external circuit, such that the ions form an intercalation compound of the LixnC type with graphite. On discharging, the lithium ions migrate back and the electrons can flow over the external circuit to the cathode.
The term. “intercalation” in the sense of the present invention refers to the intercalation of a mobile-guest species into a host lattice without destroying the structural principle of the host substance and/or the host lattice. The host lattices have layered structures, tubular structures or cage structures, for example, in which the guest substance can be intercalated in a one-, two- or three-dimensional arrangement, often with volume expansion. The intercalation of ions is associated with oxidation or reduction of the host lattice. In electrochemical intercalation, an electronically conductive host as the electrode is anodically or cathodically polarized in the electrolyte, so that anions and/or cations, optionally solvated, move from the electrolyte into the host lattice. This electron/ion transfer reaction results in a mixed conductor, which usually has a better electronic conductivity than the starting material. Electrochemical intercalation reactions are usually reversible and the mobility of the guest ions is high, in particular in host lattices having a layered structure. Intercalation performed in this way includes three basic steps: diffusion or migration of ions, which are usually solvated, to the electrochemical double layer of the host lattice, possible desolvation and subsequent transfer of the ions into free lattice sites near the surface region of the host, and finally, diffusion of the ions into the interior of the lattice.
The concept of intercalation electrodes for electro-chemical current sources has been attracting great interest again, at the latest since the rapid development of rechargeable lithium cells. Intercalation electrodes have been investigated widely since the 1970s for applications in organic and aqueous electrolyte solutions. In other galvanic elements that have already been known of for a long time, e.g., the Zn/MnO2 element and the lead battery, reduction of the cathodic oxides takes place by way of the intercalation of a proton in MnO2 and/or PbO2.
The actual breakthrough in rechargeable lithium batteries was achieved for the first time with the market introduction of a cell which completely omits metallic lithium as an anode material namely the lithium ion cell. Instead of metallic lithium, lithium ion intercalation compounds, for example, lamellar carbon, transition metal oxides or metals forming alloys with lithium are used as the negative active compounds which can reversibly take up and release lithium ions. The positive lithium ion charges are neutralized by electron uptake or release by the host material. In comparison with metallic lithium, the theoretical values for the specific charge in use of an inactive host material are usually much lower.
Since the lithium activity in the intercalation compounds—frequently also called insertion or intercalation compounds—is lower than that of the metallic lithium, i.e., lower than 1, so the electrode potential is also shifted toward less negative values depending on the charge state. However, instead of lithium atoms, lithium ions which are much smaller are used in lithium ion intercalation compounds.
The prerequisite for intercalation of lithium ions in a host lattice material is that the host lattice matrix must be able to allow the uptake of host ions not only sterically but also electronically, i.e., must have a corresponding structure of the energy bands.
The active material of the anode of a conventional lithium-ion battery is made of graphite for example. The cathode contains, for example, lithium metal oxides in a layered structured such as lithium cobalt oxide, lithium nickel oxide or the spinel LiMn2O4. Lithium-ion batteries must be completely anhydrous because otherwise the water can react with the conductive salt (e.g., LiPF6) to form hydrofluoric acid (HF). Therefore a mixture of anhydrous aprotic solvents is usually chosen.
As mentioned previously, lithium-ion batteries usually do not have the so-called memory effect and also have an extremely low spontaneous discharge or none at all. Lithium-ion batteries are able to supply power to portable electronic devices having a high power demand when these devices would be too heavy or too large for traditional lead batteries, for example, cellular telephones, digital cameras, camcorders or laptops as well as electric vehicles and hybrid vehicles. In the model building sector and in electric power tools, they have already become well established. The usable lifetime of lithium-ion batteries is several years, for example, although that depends greatly on the use and storage conditions.
Because of the positive properties of lithium-ion batteries including lithium ion accumulators as described previously, there has been no lack of attempts in the prior art to develop the technology in this respect further.
It is known from the prior art that silicon, especially in particulate form, may be mixed into a carbon or graphite matrix or otherwise introduced, for example, by gas phase deposition or the like (in this regard, cf. also the documents WO 2005/096414 A2, DE 10 2005 011 940 A1, DE 103 53 996 A1, DE 103 53 995 A1 and DE 10 2006 010 862 A1, for example). It is fundamentally known to those skilled in the art that within the context of lithium-ion batteries, silicon is mechanically degraded and amorphized due to the volume contraction and expansion which occur during charging and discharge processes and ultimately the silicon is no longer available for storage of lithium due to obviously inferior electrical contacting and destruction. The increasingly inferior electrical contacting is counteracted by the intercalation of silicon in a carbon or graphite matrix.
N. Dimovet et al., Journal of Power Sources, 136 (2004, pages 108 ff., describe a method for mechanical mixing of silicon particles approximately 1 μm in size with natural and synthetic graphite.
Kim et al., Journal of Power Sources, 136 (2004), pages 145 ff., describe a method for mechanical mixing of silicon nanoparticies with polystyrene.
Kwon et al., Chem. Commun., 2008, pages 1109 ff., describe a method for coating Si70Sn30 nanoalloys with carbon.
WO 2005/096414 A2 describes an electrode material which is produced by mechanical mixing of particles, carbon black and graphite.
An overview of the prior art can be found, for example, in Kasavajjula et al., Journal of Power Sources, 163 (2007), pages 1003 ff.
However, all the methods known in the prior art discuss only the electrical properties or capacitances of the materials.
The mechanical properties of the powder, in particular those with regard to the availability of the expansion volume for silicon are not discussed at all or are discussed only peripherally.
A number of disadvantages are associated with the silicon-containing materials proposed as the anode materials for lithium-ion battery in the prior art: a plurality of silicon-containing anode materials proposed in the prior art tend to cause mechanical degradation and amorphization so that ultimately lithium can no longer be stored due to a lack of electrical contacting. Because of the often inadequate porosity of the matrix and host structure, this matrix and/or host structure often suffers irreversible damage due to the volume contraction that occurs in the charging operation. The materials known in the prior art often do not have adequate mechanical properties, in particular do not have the corresponding strengths.